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(Upper Cambrian, Paibian) Trilobite Faunule in the Central Conasauga River Valley, North Georgia, Usa

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(Upper Cambrian, Paibian) Trilobite Faunule in the Central Conasauga River Valley, North Georgia, Usa Schwimmer.fm Page 31 Monday, June 18, 2012 11:54 AM SOUTHEASTERN GEOLOGY V. 49, No. 1, June 2012, p. 31-41 AN APHELASPIS ZONE (UPPER CAMBRIAN, PAIBIAN) TRILOBITE FAUNULE IN THE CENTRAL CONASAUGA RIVER VALLEY, NORTH GEORGIA, USA DAVID R. SCHWIMMER1 WILLIAM M. MONTANTE2 1Department of Chemistry & Geology Columbus State University, 4225 University Avenue, Columbus, Georgia 31907, USA <[email protected]> 2Marsh & McLennan, Inc., 3560 Lenox Road, Suite 2400, Atlanta, Georgia 30326, USA <[email protected]> ABSTRACT shelf-to-basin break, which is interpreted to be east of the Alabama Promontory and in Middle and Upper Cambrian strata the Tennessee Embayment. The preserva- (Cambrian Series 3 and Furongian) in the tion of abundant aphelaspine specimens by southernmost Appalachians (Tennessee to bioimmuration events may have been the re- Alabama) comprise the Conasauga Forma- sult of mudflows down the shelf-to-basin tion or Group. Heretofore, the youngest re- slope. ported Conasauga beds in the Valley and Ridge Province of Georgia were of the late INTRODUCTION Middle Cambrian (Series 3: Drumian) Bo- laspidella Zone, located on the western state Trilobites and associated biota from Middle boundary in the valley of the Coosa River. Cambrian beds of the Conasauga Formation in Two new localities sited eastward in the Co- northwestern Georgia have been described by nasauga River Valley, yield a diagnostic suite Walcott, 1916a, 1916b; Butts, 1926; Resser, of trilobites from the Upper Cambrian 1938; Palmer, 1962; Schwimmer, 1989; Aphelaspis Zone. Very abundant, Schwimmer and Montante, 2007. These fossils polymeroid trilobites at the primary locality and deposits come from exposures within the are referable to Aphelaspis brachyphasis, valley of the Coosa River, in Floyd County, which is a species known previously in west- Georgia, and adjoining Cherokee County, Ala- ern North America. A second locality has bama. Trilobite biozone associations of these produced a few identifiable specimens of the Middle Cambrian biotas are of the Glyphaspis aphelaspine Eugonocare (Olenaspella) sepa- to Crepicephalus Zones of the traditional Lau- ratum. Specimens at these two localities are rentian Middle Cambrian Series. generally complete individuals compressed Cambrian trilobites have not been described in tan mudstones. The primary locality fea- farther east- or southward from the above in the tures abundant body cluster accumulations, southern Appalachian outcrop, although the implying mass mortality by bioimmuration. Conasauga Formation is exposed in a separate The trilobite assemblage also includes the ag- fault-bounded slice in the Conasauga River Val- nostoids Glyptagnostus reticulatus, Agnostus ley (Figure 1). We report here new Upper Cam- inexpectans, and Aspidagnostus rugosus, all brian localities in this outcrop, containing the correlated to the global Paibian agnostoid southeastern-most Cambrian trilobites in North Glyptagnostus reticulatus Zone. These locali- America. The new trilobite assemblages in- ties contain the southeastern-most Late clude polymeroids of the Laurentian Upper Cambrian faunule in the Appalachians. The Cambrian Aphelaspis Zone, including Aphelas- trilobite species and carbonate-free, mud- pis brachyphasis and Eugonocare (Ole- stone lithology, lacking evidence of infaunal naspella) separatum; and coeval agnostoids of bioturbation and burrowing, suggest accu- the global Glyptagnostus reticulatus Zone, in- mulation eastward of a paleotopographic cluding Glyptagnostus reticulatus, Agnostus in- 31 Schwimmer.fm Page 32 Monday, June 18, 2012 11:54 AM DAVID R. SCHWIMMER AND WILLIAM M. MONTANTE Figure 1a— Map of Conasauga Formation Figure 1b—correlation chart showing the outcrops in Georgia with Aphelaspis Zone stratigraphic position of the Aphelaspis localities in the Conasauga River Valley faunule. (Correlation chart based on Bab- indicated, also showing the general geo- cock and Peng, 2007, and Ogg, 2009). graphic position of the trilobite beds (dis- cussed in text) in the proximal Coosa River Valley. expectans, and Aspidagnostus rugosus. in the southern Appalachians, the occurrence of In contrast with most prior reports of Upper a well-demarcated Aphelaspis assemblage with Cambrian trilobites from the Southern Appala- the globally-correlative agnostoid Glyptagnos- chians (e.g. Butts, 1926; Resser, 1938; Palmer, tus reticulatus Zone provides well-dated Upper 1962; Rasetti, 1965), specimens collected in the Cambrian sites in the south-easternmost Appa- new localities are preserved in mudstones as lachians. compressed, typically complete individuals, with intact librigenae. At Locality 1 (Figure 1a), GEOLOGIC SETTING Aphelaspis specimens are very abundant, with intact librigenae, associated in “body clusters” The localities here are in the Conasauga For- (sensu Whittington, 1997b), and commonly mation in Murray and Gordon Counties, north- surrounded by decay-gas induced iron oxide ha- western Georgia. This study area (Figure 1a) is los (Schwimmer and Montante, 2007). These in the Appalachian Valley and Ridge Province, new trilobite localities contain relatively few characterized by series of northeast-trending, agnostoids, but enough species are present (as ridge-forming Paleozoic thrust faults and trans- indicated above) to be clearly referable within forms, with relatively low valleys floored with the global Cambrian agnostoid-based stratotype less-resistant strata sandwiched between fault system (Peng and Robison, 2000; Babcock and boundaries. The mudstones of the Conasauga others, 2007; and see discussion in “Biostratig- Formation form two such valleys (Figure 1a), raphy”). In addition to extending the southeast- and the two localities discussed here lie in the ern geographic range of Upper Cambrian strata broad valley of the Conasauga River, situated 32 Schwimmer.fm Page 33 Monday, June 18, 2012 11:54 AM UPPER CAMBRIAN TRILOBITES, NORTH GEORGIA between the East Coosa and Cartersville Faults BIOSTRATIGRAPHY (Thomas and others, 2000; Thomas and Bayo- na, 2005). Locality 1 is a riverside outcrop on Trilobites from both new localities are as- the Conasauga River in Murray County, in the signed to the Aphelaspis Zone (Figure 1b), vicinity of Chatsworth, Georgia, on the eastern which is coeval with the lowermost biozone of side of the Conasauga River Valley. This local- the global of the Furongian Series (= Upper ity has an approximately 4.0 meter-thick expo- Cambrian in traditional Laurentian nomencla- sure of abundantly fossiliferous, tan-to-olive, ture: Ludvigsen and Westrop, 1985). The Con- flaggy-bedded mudstones exposed on the banks asauga sites in consideration are assigned to the of the river and in ancillary drainages. Aphelaspis Zone based on the co-occurrence of Polymeroid trilobites are very abundant at this Aphelaspis brachyphasis, and Eugonocare site, and include numerous individuals with at- (Olenaspella) separatum (Palmer, 1962, 1965; tached librigenae. Locality 2, which is now cov- Pratt, 1992). ered, was a commercial borrow-pit excavation The base of the Aphelaspis Biozone is bio- in the vicinity of Calhoun, Georgia, in southern- stratigraphically equivalent to the first appear- most Gordon County, exposing approximately ance of the agnostoid Glyptagnostus reticulatus 6.0 m of olive-green mudstones with a relative- (Babcock and others, 2005). This species is the ly sparse trilobite fauna. This locality is in the eponym of the biozone that comprises the low- southwestern-most edge of the Conasauga Riv- est unit of the global Paibian Stage (Gradstein er Valley. and others, 2005; Babcock and Peng, 2007), The Conasauga Formation in northwestern which is penecontemporaneous with the Lau- Georgia spans much of the Middle Cambrian rentian Steptoean Stage (Ludvigsen and We- (Schwimmer, 1989, Schwimmer and Montante, strop, 1985; Peng and others, 2004). The new 2007) through the lowermost Upper Cambrian Conasauga sites include three agnostoid species (Figure 1b). Collections for this report are from common to the Glyptagnostus reticulatus Zone exposures in the upper portion of the Conasau- (Peng and Robison, 2000), including G. reticu- ga Formation in Georgia. The Conasauga in its latus itself. Therefore, these Conasauga sites in- entire geographic range, extends down to the clude age-specific genera and species base of the Middle Cambrian, up through most incorporated in both Laurentian polymeroid- of the Upper Cambrian, and is mapped from and global agnostoid trilobite chronostrati- central Alabama, across northwestern Georgia, graphic zone concepts. to eastern Tennessee and southwestern Virginia (Palmer, 1971; Hasson and Hasse, 1988; Os- SYSTEMATIC PALEONTOLOGY borne and others, 2000), reaching local thick- nesses exceeding 1000 m. Across the entire Repository and Terminology exposure, the Conasauga represents multiple depositional environments forming on the ma- Specimens described and figured are curated rine shelf and in shelf-edge basins along the sa- and housed in the Cambrian Research collec- lients and recesses of the Laurentian margin of tions (CSU ) of Columbus State University, the Cambrian Iapetus Ocean. These paleoenvi- Columbus, Georgia. Descriptions of ronments include shallow-water peritidal clas- polymeroid trilobites follow Whittington tic wedges, admixed outer shelf carbonates and (1997a); agnostoid morphology also incorpo- shales, and algal carbonate shoals on the shelf- rates basic terminology from Öpik, 1967, as to-basin boundaries (Hasson and Haase, 1988). modified by
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